1. Field of the Invention
The present invention relates to a wafer polishing apparatus for polishing the surface of a wafer and, more particularly, to the structure of a retainer for retaining the periphery of a wafer.
2. Description of the Related Art
A wafer manufacturing process includes a polishing step of mirror-finishing the surface of a wafer. In this polishing step, a wafer polishing apparatus is used to polish the surface of a wafer by pressing the wafer on the surface of a polishing pad that is rotating.
The wafer polishing apparatus has a polishing table that is rotated by a drive shaft. A polishing pad is provided on the top surface of the polishing table, and a wafer retaining head for retaining and rotating a wafer is provided on the top surface of the polishing pad.
The wafer retaining head has an up-and-down driving mechanism and allows the retained wafer to be pressed on the surface of the polishing pad at a desired contact pressure. The surface of the wafer pressed on the polishing pad is polished and mirror-finished by friction with the polishing pad.
A great force is exerted on a wafer under polishing toward the outer radius of the wafer retaining head by friction with the polishing pad that is rotating. A retainer is therefore disposed on the outer region of the underside of the wafer retaining head to retain the periphery of a rotating wafer, thereby preventing the wafer from jumping out of the underside of the wafer retaining head.
The polishing pad is usually formed of viscoelastic materials. The viscoelastic materials have a viscous region whose stress is proportionate to the speed of deformation. As shown in FIG. 8, therefore, a great pressure is exerted on the periphery of a wafer W due to the compression and deformation of the polishing pad 101 when a (non-compressed) portion of the polishing pad 101, which is located ahead of the wafer W in its moving direction, goes under the wafer W. This causes a problem of a so-called outer sag in which the periphery of the wafer W is polished more than the central part thereof, as illustrated in FIG. 10.
In order to resolve the above problem, recently, the polishing pad 101 that is located ahead of the wafer W in its moving direction has been compressed in advance by pressing the retainer 102 on the polishing pad 101, as shown in FIG. 9. When the polishing pad 101 goes under the wafer W, no great pressure is exerted on the periphery of the wafer W; therefore, an outer sag can be prevented from occurring on the wafer W.
FIG. 11 shows the structure of a wafer retaining head 103 corresponding to the above-described wafer retaining head.
In the wafer retaining head 103, the wafer W and retainer 102 are brought into contact with the polishing pad 101 by applying the load of one press platen 104 to the wafer W and retainer 102. This structure can simplify the structure of the wafer polishing apparatus, whereas it causes the following problem. Due to the friction between the polishing pad 101 and the underside of the retainer 102, no desired load can be applied to the polishing pad 101 as time elapses.
When the load of the retainer 102 on the polishing pad 101 lowers, the polishing pad 101 on the underside of the retainer 102 increases in thickness, thereby causing a difference in thickness between the polishing pad 101 on the underside of the retainer 102 and the polishing pad 101 on the underside of the wafer W. The stress due to the compression and deformation of the polishing pad 101 is therefore likely to cause an outer sag on the periphery of the wafer W.
In order to resolve the above problem, a wafer retaining head 105 is used to control the load applied to the wafer W and the load applied to the retainer 102 independently of each other. The structure of the wafer retaining head 105 is shown in FIG. 12.
With the structure shown in FIG. 12, the polishing pad 101 can always be pressed by the same load even though the retainer 102 is worn out by friction between the retainer 102 and the polishing pad 101. Since, however, the retainer 102 plays a role in retaining the periphery of the wafer W as described above, the retainer 102 needs to be disposed close to the outer circumference of the wafer W. Since the retainer 102 presses the polishing pad 101, a prominence 110 arising from the polishing pad 101 is likely to interfere with the periphery of the wafer W and cause an outer sag.
In order to resolve the above problem, another wafer retaining head 108 is developed in which a groove 107 is formed at the inner radius of the retainer 102 to escape the prominence 110 and a portion of the retainer 102 which presses the polishing pad 101 and another portion thereof which retains the wafer W are separated from each other (disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2002-18709). The structure of the wafer retaining head 108 is shown in FIG. 13.
Since the wafer retaining head 108 can control the load applied to the wafer W and the load applied to the retainer 102 independently of each other, it can always press the polishing pad 101 at a constant force. Moreover, the prominence 110 that is formed at the inner radius of the retainer 102 does not interfere with the periphery of the wafer W and thus no outer sag occurs.
In the wafer retaining head 108, however, the contact portion of the retainer 102 with the polishing pad 101 is gradually worn out by friction. Consequently, the groove 107 is gradually shallowed and finally vanished.
As a result, the entire underside of the retainer 102 presses the polishing pad 101, and the prominence 110 that is formed at the inner radius of the retainer 102 causes an outer sag on the periphery of the wafer W.